Time in which 50% of the atoms of a radioactive substance transform/disintegrate (decay) to another nuclear form.

See also Half-Life, Biological Radioactive Half-Life and Effective Radioactive Half-Life.

Planck is a MKS unit of energy (expended over time to angular momentum, etc.). One Planck unit is equal to 1 joule second (J x s) or about 0.7375 foot pound second (ft x lb x s). Atomic nuclei possess an intrinsic angular momentum referred to as spin, measured in multiples of Planck's constant. The unit is named for the German physicist Max Planck.

A positron is a positively charged, with a resting energy of at least 511 keV, subatomic particle. A positron is the antiparticle of an electron, identical in mass and spin.
Positrons can be generated by positron decay or pair production.
Positron emission tomography detects positrons from the decay of radioactive tracers.

See also Beta Decay.

A proton is a basic particle located in the nucleus of an atom. The number of protons in the nucleus governs the chemical properties of that element. The positive electrical charge is equal to the negative charge of an electron and the mass is similar to that of a neutron (see also baryon). The proton belongs to the group of hadrons and is composed out of three quarks.
A hydrogen (H1) nucleus is build of one proton.
Proton beam therapy is the preferred treatment for cancers near the eye and the spinal cord.

Quantum is a discrete and the smallest natural unit of energy and momentum. Planck makes the assumption that every energy transfer on a sub-atomic level consist of small units, called quanta. The view of electromagnetic energy as photons reflects this quantization.
E = h x v
E = energy
h = Planck quantum of action = 6.6261 x 10^-27 erg sec
v = frequency.